The present invention pertains to the aircraft flight control art and, more particularly, to an automatic system for transferring aircraft trim control from the elevators to the stabilizers.
Most commercial transport aircraft in service today use the stabilizer to provide long-term or "trim" pitch control, with the elevator being used to provide short-term or maneuvering pitch control. Usually, the trim error is allowed to be held on the elevator until it is worthwhile transferring it to the stabilizer.
For single-channel autopilot elevator control, i.e., for cruise operation wherein only one of multiple autopilots is used to control flight, total control authority is typically limited to a maximum of "lg" maneuvering control in order to meet the Federal Aviation Requirements pertaining to hardover failure. This lg authority is typically most restrictive (in terms of elevator travel) at the maximum dynamic pressure in the aircraft's flight envelope and drops off significantly at lower dynamic pressures. As a result, substantially reduced autopilot maneuvering authority is available in the portion of the flight regime wherein autopilot maneuver authority requirements, primarily due to engine thrust change for control, are largest (e.g., at low airspeeds).
A satisfactory autopilot trim system should transfer elevator "trim" to the stabilizer such that:
(1) Most of the autopilot elevator authority remains available for maneuvering, and PA1 (2) The transient or "bump" which occurs when the autopilot disengages or is disengaged is maintained at a comfortable level.
In one automatic trim system known to the prior art, trim was transferred from the elevator to the stabilizer in response to the elevator being deflected beyond a threshold value for a fixed consecutive period of time, with a lower fixed value of elevator deflection used as the transfer cessation threshold.
A second prior art approach used stabilizer position to modify the previous valves of elevator used as the transfer threshold.
Both of these prior art trim systems produced nuisance stabilizer trim activity which is not only an annoyance to the flight crew, but also causes excessive wear to the stabilizer trim actuator. In addition, both of the prior art systems caused proper trim to be activated long after the trim requirement was created during certain flight conditions. In addition, both systems were known to produce annoying "bumps" when the autopilot was disengaged.
There is a long-felt need in this art, therefore, for an automatic trim system which does not suffer the disadvantages known to prior art systems.